1. Field of the Invention
This invention relates to circuit breakers which respond to sputtering arc faults as well as bolted line-to-neutral and ground faults and, more particularly, to such a circuit breaker which discriminates between sputtering arc faults and in-rush currents generated by connecting certain loads to the protected conductors.
2. Background Information
Conventional residential circuit breakers have a thermal trip device which responds to persistent overcurrents of moderate magnitude to provide a delayed trip, and a magnetic trip device which responds instantaneously to overcurrents of large magnitude. Thus, the fault current must reach a predetermined magnitude, for example ten times rated current for the instantaneous trip to occur, or the overcurrent must sustain a predetermined average value over a given time interval to implement the delayed trip. There is a type of fault, however, which may not produce either the peak magnitude required for the instantaneous magnetic trip or the sustained average overcurrent necessary for the delayed trip, yet may pose a fire hazard. This is the intermittent or sputtering arc-type of fault. Such a fault can occur, for instance, between two conductors that are in close proximity, but not touching so that an arc is struck between the conductors. This arc can produce a temperature high enough to melt the copper in the conductor. The melted droplets of copper can ignite flammable material in the vicinity. The resistance of the wire may be high enough to limit the peak current and the ac current cyclically passes through zero to extinguish the arc so that the average current is low. Thus, the conventional circuit breaker does not respond to the fault, although a hazard exists. This is especially true in the case of a stranded wire extension cord where an individual strand can be melted at a relatively low fault current.
A sputtering arc fault typically occurs near the peak of the ac voltage waveform thereby resulting in a step increase in current. This is due to the electrostatic force which causes the conductors to move toward each other. Also, if the voltage and the field are large enough a conduction may be indicated. As the current increases, the magnetic field pushes the electrodes apart producing an arc which is extinguished as the alternating current passes through zero.
The above cross-referenced patent application discloses a circuit breaker which takes advantage of this difference between the sinusoidal wave form of a line-to-neutral fault and the step wave form of a sputtering arc fault by monitoring the rate of change of current, di/dt, in the protected circuit. The di/dt signal is bandwidth limited. The selection of the band provides control of relative sensitivity of the circuit breaker to the sinusoidal overcurrents produced by overcurrent faults and step currents associated with sputtering arc-type faults. This circuit breaker includes a ground fault detector of the dormant oscillator type. The sputtering arc feature of the circuit breaker shares a common di/dt sensing coil on the neutral conductor with the ground fault detector.
The arcing wave form characterized by fast turn on to high values of current produced by sputtering arc faults is also produced by some appliances, or groups of appliances switched on simultaneously. For instance, an iron which is turned on at the peak of the voltage wave form results in a step increase in current. Also, a television receiver with a transformerless power supply turned on at a peak of the ac wave form can result in a large in-rush current. Typically, the magnitude of the in-rush currents produced by these appliances is not as large as a sputtering arc fault and, thus, the circuit breaker of U.S. Pat. No. 5,224,006 can avoid false trips by appropriate setting of the threshold level of the bandwidth limited di/dt signal.
It would be desirable; however, to be able to discriminate between sputtering arc faults and in-rush currents produced by some appliances, so that the threshold of detection of the sputtering arc wave form could be lowered to provide greater protection for the conductors to which the circuit breaker is connected.